The packaging process for high resolution linear image sensors starts with a single piece of ceramic or plastic IC package onto which an image sensor is mechanically attached and wire bonded. Optionally, a dichroic filter is attached to the imager surface, then an external wire bond light shields is attached to the IC package. The process is completed by the attachment of a cover glass with dimensions approximately equal to the IC package length and width, as depicted in FIG. 1.
The cover glass used for image sensors must be of very high optical quality and have an anti-reflection coating due to the close proximity to the image sensor where it resides, hence it is expensive. However, the actual area of glass which lies in the optical path (A.sub.a) is only a small fraction of the total area (A.sub.t). Ideally, the cover glass should be only slightly larger in area than A.sub.a in order to minimize the artifact. A single piece IC package capable of handling the smaller area cover glass would make it impossible to insert the image sensor die and wire bond it to the IC package lead frame. The cover glass is attached to the outer package surface, or sometimes a recessed surface, by activating a pre-deposited epoxy located around the perimeter of the cover glass through the application of elevated temperatures, a process which can take several hours to complete. If the cover glass is attached to the outer surface, the corners of the glass must be polished (e.g. rounded) to prevent cutting of persons involved in the assembly process. The process used to manufacture cover glass with this pre-deposited epoxy ring adds considerable cost to the cover glass--more than doubling the cost in come cases. Furthermore, the epoxy has a storage life of approximately 1 year, and glass not used by the end of this period must be discarded.
The external light shield is first machined or molded to form the aperture opening and then plated with an optically opaque material to absorb off axis and scattered light. The cost to manufacture these light shields is relatively low, but still may account for several percent of the overall packaging price tag. Like the cover glass, the total light shield area is slightly smaller than the overall IC package area, again adding costs to the packaging process. A further drawback of using an unnecessarily large area light shield (and cover glass) is that stray, or off-axis light can reflect off the outer surface of the light shield (or cover glass) and into the imaging plane, resulting in a degraded imager output. Therefore, minimizing the total area of the light shield and cover glass which is parallel to the imaging optics is a desirable goal.
The image sensor package assembly and assembly process described above has a number of drawbacks which the invention described herein addresses.